The purpose of this research is to establish reproducing colonies of germ-free (GF) C3H mice and GF Fischer rats maintained on chemically defined, water-soluble, ultra-filtered, antigen-free diets, and to study their functional and metabolic parameters. These model systems offer maximal precision and control of environmental variables in studies of nutritional requirements, basic immunology, gastroenterology, and pharmacology. Present diet formulations sustain normal growth and lifespan in C3H mice, but only marginal reproduction. The research aims first to complete the development of chemically defined diets quantitatively adequate for fully normal birth weights and weaning weights of large litters. Dietary development will be guided by plasma amino acid analysis, studies of lipid absorption, and recent reports of newer trace mineral requirements. Young which have shown normal early growth will be studied to determine physiologic and metabolic characteristics in those areas where GF status of feeding of pre- digested, low molecular weight diet might, on the basis of previous experience, be expected to cause functional shifts. The areas include cecal size and composition of contents, intestinal transit time, nutrient absorption rates, mucosal epithelium renewal rate, cholesterol- bile acid metabolism, minimum oxygen consumption, energy balance, and thyroid maturation. The immunological effects of feeding antigen-free diet will be evaluated in terms of circulating gamma globulin, plaque- forming spleen cells, and cellular response to mitogens. Research to reduce cecal distention in GF rodents will be especially centered on this problem in the GF gerbil, so as to develop a reproducing GF gerbil colony for cholesterol-bile acid studies with application to cardiovascular and colon cancer research.